Vibrating forces due to inertia forces are caused by the crank mechanism and imbalance in the rotating object.
Among the inertia-caused vibrating forces, reciprocating mass can be balanced to a certain level by selecting the appropriate cylinder arrangement and crankshaft shape. But vibration caused by unbalanced inertia is likely to remain. To improve this phenomenon, some engines are equipped with a balance shaft. The vibrating force caused by the imbalance in the rotating object is the engine’s first order vibration, regardless of the number of cylinders. The imbalance is reduced by using balance weights. The force generated by the imbalance increases in proportion to the square of the rpm, so vibrations amplify drastically as the rpm increases.
Balance shaft
Two balance shafts have been provided in recent in-line four-cylinder engines having large displacements to reduce engine vibration. Even though the crankshaft rotates at a constant speed, the speed at which the pistons reciprocate is not the same: a piston, when it is at the position shown in B, will be moving faster than the piston when it is at position A.
This will cause the upward inertial force of the piston to be greater than its downward inertial force, so the net balance of inertial forces will be in the upward direction; this will cause vibration to occur.
In a four-cylinder in-line engine, the pistons in cylinder nos. 1 and 4 are at top dead center when the pistons in cylinder nos. 2 and 3 are at bottom dead center; that is, the two sets of pistons are 180 degree out of phase with each other. For this reason, the remaining upward inertial force (mentioned above) will create a type of vibration called ” booming”, twice for each rotation of the crankshaft. To help cancel out this vibration, two balance shafts, located below or above the crankshaft, rotate in opposite directions form each other, at twice the speed of the crankshaft.